1. Field of the Invention
The invention pertains generally to the field of cathodoluminescent phosphor materials and to cathode ray displays employing them and more particularly concerns improved single particle penetration phosphors for use in bright color display cathode ray indicators.
2. Description of the Prior Art
Multicolor penetration phosphor cathode ray tubes enjoy a wide range of applications in modern display systems. In the case of avionics displays, the particular requirement of such systems are generally not met by cathode ray tubes of the types conventionally used for color television viewing. In avionics displays the system must be designed to operate under the extreme condition of sunlight falling perpendicular to the faceplate at approximately 10,000 foot candles, as well as the more typical lighting level of daytime light of approximately 100 foot candles. Display readability under high lighting levels is normally maintained by increasing the display brightness and employing a contrast enhancement device. For a given penetration phosphor screen, however, increased brightness, which is obtained by increasing the beam current density, will lead to a decreased screen lifetime. This fact, coupled with limitations in the coulomb ratings, luminous efficiencies and designed operating voltages for a state of the art multicolor penetration phosphor has led to the employment of directional filters in order to simultaneously meet display readability requirements and obtain satisfactory screen lifetimes. The use of directional filters, however, has the disadvantage of requiring the viewer to carefully position his head with respect to the display in order to take advantage of the improved light transmission.
In prior art embodiments phosphors having both wide and narrow emission spectra have been used in combination with selective narrow bandpass filters, which do not suffer the disadvantage described above for the directional filters. The use has, however, been limited by the lack of a penetration phosphor with acceptable cathodoluminescent properties, since in addition to filtering out unwanted wavelengths of light such as is contained in sunlight, these filters may also filter out a large portion of the phosphor's emission.
While several kinds of color television cathode ray tubes are currently available, including the older type with a mask with round holes, the inline slot mask color tube, and the recent slit mask color tube, all of these use multiple guns and complex electron beam focusing and scanning arrangements and are generally not suited for use in information displays, especially where random deflection is needed. Resolution is poor, and sensitivity to external magnetic fields is undesirably high. Because they require multiple cathode and multiple electrode systems, sensitivity to shock and vibration may also be a problem.
While originally conceived for use in color television receiver displays, the penetration phosphor color tube and the principles it employs offer several advantages for use in information displays.
Conventional penetration phosphor cathode ray tubes in their most prevalent form exploit the ability to control the depth of electron penetration into the phosphor screen of the CRT by adjusting the voltage of electron beams incident upon the multilayered phosphor system. Thus, at low voltages, only the phosphor closest to the electron source is excited, yielding an output color corresponding to its emission. At the highest voltages, inner layers are also excited yielding an output color that is determined by the relative emission intensities from the contributing phosphors. Intermediate voltages then give rise to different relative emission intensities and hence different colors.
Of the various possible approaches for constructing the requisite multilayered phosphor system, those utilizing multilayered powdered particles have received considerable attention for reasons of enhanced luminous efficiencies or ease of subsequent tube manufacture. One early version of a mixed two component system using red and green emitting phosphors involved the formation of a non-luminescent "onion skin" on the surface of a green emitting ZnS:Cu powder particles. This dead layer green (DLG) component was then mixed with commercially available red emitting phosphor, allowing the preparation of a multicolor phosphorous screen using the same procedure employed in monochrome tube preparation. ZnS:Cu powder is not ideally suited for use in high contrast displays because of its reduced luminous efficiency under the high current density conditions found in these displays. Furthermore, it is not ideally suited for use with selective filters because of the broad band nature of its emission as discussed above.
In Another approach an efficient penetration phosphor consisted of a Zn.sub.2 SiO.sub.4 :Mn core particle covered with a non-luminous layer on top of which was a coating of small red emitting YVO.sub.4 :Eu particles. These penetration phosphors, however, also use a broad band green emitting phosphor which reduces their suitability for use with selective, contrast enhancement filters.
In another embodiment of a single particle penetration phosphor system containing only line emitting phosphor components, the preparation involved a controlled sulfidization or R.sub.2 O.sub.3 :Pr, where the R could be yttrium or gadolinium, particles to yield a core of red emitting R.sub.2 O.sub.3 :Pr in a contiguous surface layer of green emitting R.sub.2 O.sub.2 S:Pr. Although the narrow band aspect of the component phosphor emissions makes this system well suited for use with selective filters, the availability of alternative red and green emitting phosphor components with superior cathodoluminescent efficiencies and color saturation provides a opportunity for improvements in system performance. The present invention provides for a single particle penetration phosphor system utilizing phosphor having superior cathodoluminescent efficiencies and color saturation characteristics to provide for improvement over prior art penetration phosphor systems.